In order to couple light from a first optical fiber to another optical fiber, it is necessary to perturb the light propagating in the core of the first fiber so that it radiates out of the bound modes. A variety of techniques are available for perturbing the light propagating in the core of an optical waveguide and more particularly an optical fiber; some examples are fiber tapering, fiber bending and in-fiber gratings. The technique for perturbing the light propagating in the core of an optical waveguide of interest in this invention is by using gratings photoimprinted in the optical waveguide core. The advantage of photoimprinted gratings in the fiber core for perturbing the light propagation is that the process is noninvasive; that is, the fiber core and its cladding are not damaged. However, the invention is not restricted to gratings fabricated using light. Alternatively, ratings in the form of periodic corrugations formed using wet or dry etching may also be used.
U.S. Pat. Ser. No. 4,474,427 (K. O. Hill, B. S. Kawasaki, D. C. Johnson and Y. Fujii, "Optical fiber reflective filter", filed May 7, 1979, issued Oct. 2, 1984) disclosed that most glass optical fibers are photosensitive and that light can be used to write permanent refractive index gratings in the core of optical fibers. These gratings subsequently became known as "Hill gratings". U.S. Pat. Ser. No. 4,807,950 W. H. Glenn, G. Meltz, and E. Snitzer, "Method for impressing gratings within fiber optics", filed Nov. 19, 1987 and issued Feb. 28, 1989) showed that the "Hill gratings" could be formed by irradiating the fiber from the side with two intersecting coherent ultraviolet light beams. The two overlapping ultraviolet light beams interfere producing a periodic interference pattern that writes a corresponding periodic index grating in the core of the optical fiber. The technique called the transverse holographic technique is possible because the fiber cladding is transparent to the ultraviolet light whereas the fiber core is highly absorbing to the ultraviolet light. Another technique for photoimprinting index gratings in the core of an optical fiber is the phase mask technique which was disclosed in U.S. Pat. No. 5,367,588 (K. O. Hill, B. Y. Malo, F. C. Bilodeau, and D. C. Johnson, "Method of fabricating Bragg gratings using a silica glass phase grating mask and mask used by same", Filed Oct. 29, 1992, issued Nov. 22, 1994). The phase mask is a flat slab of silica glass that is transparent to ultraviolet light. On one of the flat surfaces, a one dimensional periodic surface relief structure is etched using photolithographic techniques. The shape of the periodic pattern approximates a square wave in profile. The optical fiber is placed almost in contact with the corrugations of the phase mask. Ultraviolet light which is incident normal to the phase mask passes through and is diffracted by the periodic corrugations of the phase mask. Normally, most of the diffracted light is contained in the 0,+1 and -1 diffracted orders. However, the phase mask is designed to suppress the diffraction into the zero-order by controlling the depth of the corrugations in the phase mask. In practice the amount of light in the zero-order can be reduced to less than 5% with approximately 40% of the total light intensity divided equally in the .+-.1 orders. The two .+-.1 diffracted order beams interfere to produce a periodic pattern that photoimprints a corresponding grating in the optical fiber. The phase mask technique has the advantage of greatly simplifying the process for photoimprinting Bragg gratings in optical fibers.
In most applications the Bragg gratings are photoimprinted such that the index perturbations are normal to the longitudinal axis of the optical waveguide. Such an index grating in the core of a singlemode optical waveguide has the characteristic of reflecting light at the Bragg wavelength, .lambda..sub.B given by .lambda..sub.B =2n.sub.eff .LAMBDA. where n.sub.eff is the effective refractive index of the optical waveguide and .LAMBDA. is the pitch or period of the perturbations in the index grating. The light at the Bragg wavelength is reflected from the grating and coupled back into the bound mode of the optical fiber so that it is now propagating in the opposite direction. Light at wavelengths different from the Bragg wavelength pass through the index grating unperturbed.
If the gratings are photoimprinted in the core of the optical waveguide such that index perturbations are tilted or slanted to the optical waveguide axis, the light reflected at the Bragg wavelength is coupled into higher order modes or into radiation modes. The application of tilted gratings for mode converters was first demonstrated by Hill et al. (K. O. Hill, B. Malo, K. A. Vineberg, F. Bilodeau, D. C. Johnson, and I. Skinner, "Efficient mode conversion in telecommunication fibre using externally written gratings", Electronics Letters, Vol. 26, No. 16, pp. 1270-1272, Aug. 2, 1990). The first demonstration of the use of tilted grating to couple light to the radiation modes was reported by Meltz et al. (G. Meltz, W. W. Morey, and W. H. Glenn, "In-fiber Bragg grating tap", Conference on Optical Fiber Communications, OFC'90, San Francisco, Calif. Jan. 22-26, 1990. Proceedings ofthe OFC'90, Paper TUG1, pp. 24, Jan. 23, 1990). Tilted gratings are the subject of U.S. Pat. Nos. 5, 546,581 and 5,511,083 and have applications as fiber taps and fiber polarizers. It is the capability of tilted or slanted index gratings to couple light to the radiation modes that is relevant for this invention.
A tilted grating in the core of an optical waveguide can couple light from the bound modes to radiation modes; the characteristics of radiated light are important to this invention. For a grating having a period .LAMBDA. (wave number K.sub.g) and tilted at an angle .theta. (.theta. is the angle the wave vector K.sub.g makes with the fiber longitudinal axis), monochromatic light at a wavelength .LAMBDA. (propagation constant .beta.) is scattered at an angle 2.theta. where .theta. is given by the expression 2.beta. cos .theta.=K.sub.g. This expression is easily obtained by considering the light as plane waves and momentum must be preserved between the incidence wave vector .beta..sub.inc, the scattered wave vector .beta..sub.sca and the grating wave vector K.sub.g with .vertline..beta..sub.inc .vertline.=.vertline..beta..sub.sca .vertline.=.vertline..beta..vertline.. Light having a different wavelength will be scattered at a different angle. Consequently, the tilted grating taps light over a broad range of wavelengths. The distribution of the scattered light is also of interest. In the scattering plane, monochromatic light scattered at an angle 2.theta. has a very small angular spread about 2.theta., typical the angular spread is less than a milliradian. However, in the plane normal to the fiber axis the angular azimuthal .phi., spread around the fiber axis is much larger, typically of the order of 20 degrees. This description of the radiation of light scattered by a tilted grating in an optical fiber has been somewhat qualitative; a more rigorous treatment of the problem can be found in a paper by Erdogan and Sipe (T. Erdogan and J. E. Sipe, "Tilted fiber phase gratings", Journal of the Optical Society of America, Vol. 13, No. 2, pp. 296-313, February, 1996). An objective of this invention is to take light scattered into the radiation modes by the tilted grating and couple it into another optical waveguide.
An alternative noninvasive method for perturbing the light propagating in the core of an core fiber is to photoimprint the grating using ultravolet with intensities greater than 1 joule/cm.sup.2. (See for example, B. Malo, D. C. Johnson, F. Bilodeau, J. Albert, and K. O. Hill, "Single excimer pulse writing of fiber gratings using a zero order nulled phase mask: Grating spectral response and visualization of index perturbations", Optics Letters, Vol. 18, No. 15, pp. 1277-1279, Aug. 1, 1993). At these high intensities a different nonlinear photosensitive mechanism begins to dominate and a periodic index perturbation is written that is localized to the core/cladding interface. This method for writing index gratings has the advantage that they are formed in a single exposure to high intensity ultraviolet light. Since these index perturbations do not extend throughout the core they cause light in bound modes to radiate to the free space modes.
Another technology relevant to this invention is the planar optical waveguide implementation of "Dragone couplers". The principles underlying these couplers are described in a paper (C. Dragone, "Efficient N X N star couplers using Fourier optics", Journal of Lightwave Technology, Vol. 7, No. 3, pp. 479-489, Mar. 1989) and U.S. Pat. Nos. 4,904,042, 5,039,993 and 5,136,671. Briefly, the paper and patents teach that efficient coupling between two arrays of N elements can be realized in free space. In practice the free space region is a planar slab waveguide with curved entrance and exit ends. The two curved ends are separated by a distance corresponding to the radius of curvature of the slab ends. Along the curved entrance port of the planar waveguide an array of N optical waveguides are regularly spaced. A similar array of N singlemode optical waveguides are positioned along the output port of the planar optical waveguide. Light launched into one of the singlemode entrance fibers is guided to the planar optical waveguide where it radiates into free space. The free space region has only 2 degrees of freedom since the planar optical waveguide will continue to guide the light in the 3.sup.rd dimension. The radiating light illuminates uniformly the N singlemode waveguides positioned on the far side of the slab waveguide. This receiving array is far enough away that the elements are in the far field of the radiation pattern. Using this method, light in a single fiber is divided among several fibers.
It is an object of this invention to a provide a noninvasive means for perturbing the light propagating in an optical waveguide and for coupling the radiated light into another optical waveguide. In the preferred configuration, the optical waveguides have the form of monomode optical fibers.
It is an object of the invention to perturb the light in an optical waveguide using a grating in or in close proximity to the waveguide core thereby causing the light in the core bound modes to radiate into the free space modes. The preferred means for fabricating the grating are noninvasive using photosensitivity.
It is an object of the invention to capture efficiently the light radiated out of the core of the optical waveguide by placing the optical waveguide in close contact to the circularly curved entrance end of a planar optical waveguide that guides and concentrates the captured radiated light on a distal point located one radius of curvature away on the circularly curved exit end of the planar optical waveguide where the concentrated light is coupled into another optical waveguide.